Zero Boil-Off Tank Experiment to Characterize Pressure Control Behavior in Microgravity

The Zero Boil-Off Tank (ZBOT) Experiment investigates zero boil-off pressure control strategies in microgravity. The primary objective is to validate and verify numerical models used to design more efficient cryogenic storage tanks for use in long-duration space missions. A key challenge of the experiment is to characterize and control the fluid flow and energy balance in the experiment system so they can be used for boundary conditions in the models. In this manner the performance of the numerical models can be directly compared to experimental results in microgravity and improved to extrapolate beyond the experimental boundaries. To date, a ground-based breadboard system has been developed with the primary objective of verifying flight concepts in the areas of fabrication and thermal performance. The breadboard consists of an acrylic fluid test tank that is thermally isolated from the outside environment. Heaters on the tank introduce controlled energy into the fluid, a room temperature refrigerant used as a cryogenic fluid simulant, whose temperature and pressure rise is accurately measured over time. A mechanically pumped fluid loop circulates the refrigerant in the chamber to distribute heat and control pressure, and velocity flow fields are measured using Particle Image Velocimetry (PIV). The test tank is constructed of stainless steel and acrylic materials. The transparent acrylic portion of the tank allows velocity flow field measurements, at rates of 1 μm/s to 2 mm/s using PIV. The lower stainless steel portion of the test tank provides a structural mounting platform while facilitating the installation of fluid fittings and a pressure transducer. The test tank is mounted within a sealed vacuum jacket to limit convective and radiative heat transfer. The enclosure is constructed of aerospace grade aluminum that has been electroless nickel coated. The internal surfaces have been highly polished for low emissivity to reduce radiative coupling between the test tank and vacuum enclosure; temperature matching the jacket to the tank further reduces the radiative heat loss. Hermetic connections provide fluid line and sensor access to the test tank. Strip heaters mounted to the test tank and vacuum enclosure allow precise power input. Eighty-seven high precision RTDs are mounted internally and externally to the test tank and vacuum jacket to provide a detailed mapping of the system's thermal profile.